When Irresistable Faefolk moves to an unoccupied battlefield and uses its effect to move an enemy unit to that battlefield, who is the attacker and defender?

When you move Irresistible Faefolk to an unoccupied (uncontrolled) battlefield and use its ability to move an enemy unit to that same battlefield, you are the attacker and the enemy unit is the defender.

According to RiftJudge FAQ #9581, #9665, and #9714, the determination of attacker and defender designations is based on which player applied the "Contested" status to the battlefield first (Rule 459.2.b.1).

Sequence of Events

1. Move: You move Irresistible Faefolk to an unoccupied battlefield. This applies the "Contested" status to that battlefield. Because you are the player who applied this status, you gain the Attacker designation.
2. Trigger: Irresistible Faefolk's ability ("When I move to a battlefield...") triggers. You place this ability on the chain.
3. Resolution: When the ability resolves, you move an enemy unit to that same battlefield.
4. Combat: Because units from opposing players are now present, the showdown transitions into a Combat Showdown.

Because you were the one who applied the "Contested" status to the battlefield first, you remain the attacker, and your opponent's unit is the defender, even though your ability was the one that moved their unit into the combat.